The present invention relates to a method of making a ceramic body for machining tools, rock drilling tools, wear parts, corrosion-resistant articles, heat-resistant parts and bodies for construction purposes. The method is particularly useful for making of ceramic bodies which for some reason, e.g., the outer shape, cannot directly be compacted to final shape by uniaxial pressing.
Ceramic bodies can be produced by injection molding or slip casting followed by sintering. It is also possible to produce ceramic bodies by tool pressing and sintering. As for ceramics as well as for other hard and wear resistant materials, the ultimately desired shape must as far as possible be obtained before the sintering since it is very expensive and, in some cases, not possible to grind to final shape. Grinding must be performed with diamond tools. In certain cases it is not even possible to grind all faces. For ceramic bodies, shaping is preferably made directly to final shape. If shaping of the ceramic body must be performed, it is preferred to do this directly after the forming, e.g., tool pressing or after a presintering which takes place at a temperature at which the ceramic body has a certain strength but has not sintered so much that the strength becomes too high and the shaping that much more difficult. Another method being used for the making of ceramic bodies is cold isostatic pressing followed by sintering. A shaping operation may possibly be applied before the final sintering in order to adjust the dimensions. The method is relatively expensive, however, and not suitable for small parts made in large volumes.
If small parts are to be made in large volumes, tool pressing is often a good method. Many parts then carry the relatively high cost for the pressing tool. Tool pressing of such parts, however, has some limitations. Shapes which are too complex cannot be made according to this method. It is in addition difficult to compact parts with large height/width-ratios and at the same time obtain even density in the whole part. During sintering, the part may shrink unevenly or porosity may be formed in some sections of the body. Porosity and/or cracks may appear if the geometric shape is complex. In certain cases, bodies with complex geometry can be produced using collapsible tools in which the die after the pressing is divided in order to expose the compacted body. Such tools are, however, very expensive and sensitive to the high compacting pressures used when producing ceramics and other hard alloys.
It is known through U.S. Ser. No. 07/425,237, now U.S. Pat. No. 5,275,633 and Ser. No. 07/687,676 (our references 024000-692 and 024000-798, respectively) how to manufacture a cemented carbide body with complex geometry by sintering together the body from simpler parts to a body with desired complex geometry. The sintering is performed usually at atmospheric pressure or lower and the joint is in general not visible and therefore the strength is completely comparable with that of a directly compacted body.
Ceramic materials are often difficult to densely sinter due to the fact that many ceramic systems do not contain any liquid phase during sintering. It is more difficult using solid state sintering to obtain fully dense materials due to the covalent nature of the ceramic bonds in, e.g., Si.sub.3 N.sub.4. In many cases, additions are made to the ceramic mixture (matrix) to facilitate the sintering, inhibit grain growth, or increase the strength and/or performance, e.g., the addition of MgO to Al.sub.2 O.sub.3 -ceramics.
Addition of needle-shaped single crystals (whiskers) gives particularly good properties to ceramic composites based usually on Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4. These crystals are preferably carbides, nitrides, carbonitrides, carboxynitrides and/or oxides of metals with refractory character such as the metals of group IVb (the Ti-group), Vb (the V-group) and VIb (the Cr-group) of the Periodic Table and the metals B and Si. The length to diameter ratio should be more than 5. Also, plate-shaped crystals with a length-width/thickness ratio greater than 5 have been shown to give good properties. Particularly good properties have been obtained with mixtures of needle-shaped and plate-shaped crystals. It has also turned out that materials with particularly good properties can be obtained by mixing in large and small particles, plate- and/or needle-shaped crystals of the above-mentioned type in the ceramics.
It is difficult to sinter composites with additions of needle- or plate-shaped crystals to high relative density, more than 98%, because it is difficult to homogeneously compact powder bodies with such additions. At additions of more than about 10% by volume, special sintering methods have to be applied in order to obtain a good result.
Uniaxial pressure sintering (hot pressing) is such a method where the ceramic can be compacted to high relative density. The method presupposes subsequent machining (cutting, grinding) after the sintering which is expensive and in some cases gives bodies with poor performance.
Ceramic bodies can also be made with so-called glass encapsulated HIPing (HIP means Hot Isostatic Pressing) where the bodies are embedded in molten glass acting as a pressure transmitting medium during sintering as disclosed in, e.g., U.S. Pat. No. 4,446,100. This method is particularly suited for the above-mentioned ceramic composites with larger additions of needle-shaped and/or plate-shaped single crystals. During sintering, the temperature is increased such that the glass begins to soften and forms a tight layer around the bodies. After that, the pressure in the furnace is increased to 50 MPa (Ar, N.sub.2, etc., gases are possible to use) or more and the temperature is increased to sintering temperature. Silicon nitrite-based ceramics or ceramic composites can advantageously be made in this way. Alumina-based ceramics or ceramic composites must be protected from influence of the glass and this can be done with the aid of one or more protective layers. In certain cases, an innermost layer is used which serves as a release agent (herein referred to as relief layer). This layer may comprise fine-grained BN.
U.S. Pat. No. 4,579,703 discloses the manufacture of a ceramic body, a turbine wheel of Si.sub.3 N.sub.4, where is formed at least two parts of which at least one is a shaped powder body formed from powder of a ceramic material mixed with a plasticizer. The individual parts are assembled together into a body of desired shape, the assembled parts are surrounded with a gas-impermeable layer of, e.g., glass. Finally, the assembled parts are HIPped to form a dense, homogeneous body.